Ring gears have long been used in vehicle differentials for transferring torque to a wheel axle from a rotating drive shaft and commonly have pitch diameters ranging up to 18 inches or more in heavy duty truck differentials.
Due to characteristically large size and the criticality of such application, ring gears are commonly forged from steel rather than cast so as to provide them with strength and durability.
Over the years, the art of forging gears, including ring gears, has advanced to the point where they are able to be "near-net" forged meaning that the teeth are able to be formed to less than about 0.070 inch of the finished dimensions desired for the functional surfaces such as, for example, disclosed in U.S. Pat. No. 4,761,867 assigned to the assignee of the present invention and the disclosure of which is incorporated herein by reference.
Ring gears of the type herein concerned have a plurality of teeth either of the spiral bevel type or hypoid type well know to those skilled in the art that are circumferentially evenly spaced about the central rotational axis of the gear and face in a direction acutely away therefrom. Each tooth features a pair of spaced-apart side walls extending from and following opposite edges of a bottom surface extending therebetween defining a tool path having a median radius of curvature "R" in a plane that is substantially parallel to the bottom surface of the tooth and substantially transverse to the rotational axis of the gear.
Ring gear teeth characteristically have what is known in the trade as Gleason Formate geometry, i.e., a tooth having straight side walls extending angularly away from each other from opposite edges of a bottom surface extending therebetween along a tool path having a median radius of curvature "R" in a plane substantially parallel to the bottom surface and transverse to the rotational axis of the ring gear.
Heretofore, such ring gears have been machined with specialized Gleason equipment having a machining tool featuring formed cutting blades or teeth deployed about and extending transversely from the perimeter of a rotary plate having a diameter of 2R. Although recently improved, such tools have been generally expensive; are difficult to sharpen; are slow in operation; and characteristically require long set-up times.
Ring gears are commonly forged from low to medium carbon and alloy steel such as AISI 8620A, 8622A, 8625A, 8822A, 4817H and 9310A having a carbon content of about 0.05% to about 0.5% by weight. It has also heretofore been standard practice to subject the forged ring gear to normalizing heat treatment or other microstructure modification processes after forging and before machining in order to enhance machinability by promoting more uniform grain size and essentially eliminating Bainitic Ferrite or "Widmanstatten" grain structure adjacent the surface to be machined.
In view of the foregoing, a need has existed for some time to provide a method for machining forged and/or rough-cut ring gear teeth that utilizes conventional equipment, is faster, is less expensive, and eliminates the practice heretofore of normalizing or otherwise modifying the ring gear microstructure prior to machining.